toolchain: reorganize target gcc options

* Move target gcc language selections to the package menu.
* Disable objc & fortran options unless target gcc is selected.
* Rename the "native toolchain" option to gcc.
* Disable target gcc unless we're using a buildroot toolchain.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>